1. Field of the Invention
This invention relates to a shield of an earth magnetic field which affects convergence characteristic in a cathode ray tube (CRT), and more particularly to a device for shielding an earth magnetic field which passes through a neck of the CRT and affects an electron gun to prevent the convergence of electron beams.
2. Description of the Prior Art
Typically, a cathode ray tube (CRT) includes a face panel, a funnel and a neck. Electron beams of red (R), green (G) and blue (B) emitted from an electron gun mounted on the neck are deflected by a deflection yoke provided on the periphery of the funnel and are landed on the corresponding R, G, and B phosphor screen to produce a desired picture. In this case, the CRT is affected by an earth magnetic field to change the traces of the electron beams, so that a raster position is changed and purity and convergence characteristics are deteriorated. Accordingly, an inner shield is provided in the funnel of the CRT to shield the earth magnetic field.
When the inner shield is positioned in the magnetic field, magnetic dipoles in the inner shield are orderly arranged by its spin moment and the inner shield becomes magnetized. The magnetic force generated from the magnetized inner shield counterbalances or interferes the earth magnetic field, whereby the affection on the electron beams in the CRT by the earth magnetic field is merely decreased by 50%.
Shielding effect of the inner shield in the CRT depends on its component. Generally, the component of high magnetic permeability and low coercive force has good shielding ability.
However, the inner shield of any good component cannot decrease the affection of the earth magnetic field in the CRT more than 50%.
Accordingly, the present invention is directed to a device for shielding an earth magnetic field that substantially obviates the problems due to limitations and disadvantages of the related art.
It is an object of the invention to provide a device for shielding the earth magnetic field permeating through a neck of a cathode ray tube (CRT) to improve purity, raster and convergence characteristics in the CRT.
The device for shielding the earth magnetic field according to the present invention includes a tube type shielding device, the device made of a material having a high magnetic permeability and low coercive force, and the device having four evenly spaced extension parts formed on the periphery thereof along the CRT axis.
The device may be made of a Nixe2x80x94Fe permalloy comprising Ni (nickel) more than 50 weight %, preferably 78.5 weight %. The Nixe2x80x94Fe alloy has a good magnetic force permeability and good working characteristics, such as wear resistance, ductility and malleability.
The device has an inner diameter to be mounted on the outer surface of the neck. Alternatively, the device may be mounted on the neck by means of a both-sided adhesive tape. The device may be also mounted on the neck by using a thermal contraction tube. The thermal contraction tube is contracted by heating after applying the thermal contraction tube on an outer surface of the device and neck.
A permanent magnet may be provided on the device to set up a magnetic field. Preferably, the magnetic force of the permanent magnet is between 1 mG (gauss) and 10 G. The permanent magnet may be the type of a ferrite or a rubber magnet and provides a magnetic field to decrease the affection by the earth magnetic field on the neck of the CRT.